10 replies to this topic

[The Otter]

Hi guys,

I've recently picked up a set of twin HS2 carbs for my 998, and have spent the past couple of evenings dismantling the setup and fathoming exactly how everything connects up and how it all works. I've got pretty much everything worked out now, but I wanted to check a couple of things with anyone else running twins like mine.

Firstly is the connection for the crank case breather on the tappet chest cover, these carbs don't have any intake pipes on the inside of the carb bodies as I've seen on some sets, and there is a pipe adaptor screwed in to a hole in the balance pipe of the manifold. I'm assuming this would be okay for the breather pipe connection? Adaptor shown in pic below:



Secondly, how would I go about connecting up my vacuum gauge? On the single HS4 I have at the moment I've got a replacement carb spacer with a little barbed pipe connector screwed into it (bought from minispares). I have found that minispares do these for HS2's as well ( http://www.minispare...ty=pb&pid=34394 ), and even mention about having to enlarge the mounting holes for twins, but what I don't know is how I should connect it up. Should I just buy one of these spacers and use a standard spacer on the other side? Should I buy two of these barbed connection spacers and connect the two via a t-piece, then in to the gauge? Should I be using two vacuum gauges, one for each intake port? I'm sure this is a silly question to some of you, but I don't know what's best!

Cheers, Sven.

[cooperrodeo]

I believe in the original system that the breather pipe was connected into the air filter box. Later cars had it connected to the inlet manifold via a Positive Crankcase Ventilation valve (PCV) so as to limit the amount of suction. The PCV needed to be used in conjunction with an oil filler cap which allowed air into the rocker cover to provide the crankcase ventilation. Do not connect the breather directly to the inlet manifold as there is far too much suction without a PCV. Even later carbs had the pipe on the side for the breather connection. The pipe was carefully placed so that the suction was controlled by the position of the throttle butterfly.

The other connection on your inlet manifold was for a vacuum brake servo, but could be used for your vacuum gauge if you have a suitable connector.

[bmcecosse]

You are better off with the single HS4 on a GOOD inlet manifold than these quite old small bore SUs.....

[The Otter]

Thanks for the input cooperrodeo, it's great that there's people like you that know these things, otherwise I'd have gone straight along with connecting the pipe directly to the manifold! Minispares sell those PCV valves so I'll add that to the annoyingly expensive shopping list of bits that I need for these carbs! I'll be using the other manifold connection for my brake servo, so I thought the easiest way to connect the gauge up would be to do something with those spacer adaptors, but I don't know what the best way would be to connect the spacer (or spacers) to the gauge (as I described in my initial post).

Don't worry bmceccose, I've researched enough about switching to twin carbs and heard and read lots of different opinions and I'm 100% settled on having twins.

Now, if anyone has any more suggestions for the vacuum gauge connection?

[liirge]

yer the spacers are good, also increases the length of your inlet tract, a very good thing for torque on twin SU's.
I would take a vaccum line of each spacer, and then T them together into one pipe and attach that to the vaccum gauge of your choice.

I'm glad you've decided to stick with the twins it's a lovely setup, irrespective of what some people say on here, as most of them haven't matched the components properly, or used a knackered set etc. etc.

[bmcecosse]

It would be good to hear your before and after report if you fit these. Is your single carb on an alloy manifold at the moment - or the power-robbing combined inlet/exhaust manifold?

[The Otter]

Brilliant thanks liirge That was one of the ways I was thinking of doing it, but wasn't sure if putting two connections in to one would double the pressure or anything like that, and cause it to over read on the gauge. I'll try it anyway, if it doesn't work then I'll try something else!

Yes I'm looking at doing this properly, a comprehensive re-build kit, all the right connections and such you know, no bodges. I was also planning on purchasing MED's stub stack and ITG filter kit:

http://www.med-engin...ub-stack-system

I'm also planning on modifying the butterfly valves as well, as per the Vizard yellow bible, as it made a noticeable difference on my single HS4 and costs nothing to do!

bmcecosse, yes my HS4 is fitted to a minispares alloy inlet manifold. From what I've read and understand about twin HS2's, I may lose out on power a little at high revs, but gain noticeable torque in the lower rev range. It'll be a while before I get them fitted to the car, but I will be sure to let you know my opinions on them once they're on!

[liirge]

What is the rest of the engine spec on your 998? this will decide how the twins will affect your setup..

going back to the gauge, no you won't double the pressure or anything, the only reason i say take a vacuum from both tracts is to get a more accurate vacuum figure across all 4 cylinders. I mean it doesn't matter too much as there is a balance pipe in action on that manifold, and a fairly sizable one at that!

The pipe is used to allow for easier setup, and to allow for slight changes in tune between the carbs, isn't great for air flow profile, but makes life on the owner a little easier; not that I'm saying twins are a nightmare and fall out of tune quicker than a single carb, as thats just pure *******!

[The Otter]

At the moment I've got a single box RC40 exhaust with a maniflow cooper exhaust manifold, the single HS4 on an alloy inlet manifold, a correctly skimmed 12G295 head, and (not sure if this is relevant for performance...) an Aldon electronic ignition system (the one that replaces the points in the dizzy). I was planning on maybe upgrading the cam and rebuilding the engine and box in the summer if I have the time and funds, as the engine has just ticked over to 95,000 miles but it's still going very strong! I do an oil change every 3000, 4000 miles or so and try to look after it as best as I can as it's my only car and I do a fair few miles in it.

[liirge]

Its essentially a Cooper 998 engine, for all intents and purposes. nice engine there, won't be massively revvy so you won't lose any top end, the twins will make an excellent addition to the engine you will see a good torque and low down pick up increase. If you get a camshaft I would get something like Kent 266, Swiftune SW5-07 or the old steady MG Metro camshaft.

Edited by liirge, 18 January 2012 - 10:37 AM.

[The Otter]

It would be good to hear your before and after report if you fit these.

Well there's definitely an improvement, have a look at my project thread to see the power and torque curves from the rolling road session.

http://www.theminifo...-mk1-fraudster/

Sven.

Edited by J322OverTheTop, 21 March 2012 - 11:51 PM.